The invention concerns the field of brakes, and of motor vehicle brakes in particular. More precisely, the invention concerns a parking brake actuator which can be operated by an electric motor.
Actuators, operated by electric motors, for parking brakes (also called holding brakes) are used in motor vehicles. The advantage of this realization compared with brakes which are operated exclusively by cables or hydraulics is the fact that the driver does not have to apply great muscular force to operate the brake. A parking brake which can be operated electrically can also be combined well with modern vehicle electronics.
Such parking brakes are commonly operated by means of an actuator, which usually has an electric motor with gearing connected downstream from it. The gearing can consist of several stages, and various gearing technologies can be used. For instance, gearing stages in the form of belt drives, worm gear pairs, cylindrical gearing or planetary gear sets are known. The advantages of using a planetary gear set are its compact construction, the high degree of reduction per stage, and the possibility of transmitting a high torque. On the other hand, the disadvantage is that because of the higher number of moving parts per stage compared with other forms of gearing, increased development of noise must be expected. Also, the planetary gear set is consequently more expensive to produce, and potentially its efficiency is less advantageous.
Usually, planet wheel bearing devices for planetary gear set stages which are loaded by high torques are manufactured separately from the planet carrier, and joined non-positively to the planet carrier in a manufacturing step. The generally preferred joining technique here is mating, in which steel bearing devices are fitted into recesses of a planet carrier. A disadvantage here is that the mating hole and if appropriate the corresponding mating surface must be manufactured with great precision before the parts are joined to each other. Manufacture of such parts is expensive, and requires increased attention to quality control, resulting in increased unit costs.
Additionally, brake actuators are exposed to increased requirements regarding resistance to impacts, vibration, corrosion and increased temperature. In all operating conditions, the mechanical resistance of the gearing should be constant as far as possible, to be able to provide reproducible brake application forces.
This invention is therefore based on the feature of providing an optimised actuator for an electrical parking brake.